Process of manufacturing a co-ground calcium carbonate material of the GCC and PCC type with a specific steepness factor, obtained products and their uses

ABSTRACT

An object of the present invention is to provide a process to obtain a calcium carbonate material comprising GCC and PCC, with a specific steepness factor (defined as d 30 /d 70 ×100, where d x  is the equivalent spherical diameter relative to which x % by weight of the particles are finer) of at least about 30, preferably of at least about 40, and most preferably of at least about 45, in a cost efficient manner, wherein GCC and PCC are co-ground, possibly with at least another mineral material. An other object of the present invention lies in the obtained co-ground calcium carbonate material in the form of an aqueous suspension and in the form of a dry product. An other object of the present invention lies in the uses of such products in any sector making use of mineral materials, and notably in the paper, paint and plastic industries.

It is an object of the present invention to provide a process to obtaina calcium carbonate material comprising GCC (ground calcium carbonate)and PCC (precipitated calcium carbonate). Such material is appropriatefor use in a number of domains, for example in the paper industry.

It is also an object of the present invention to provide a process toobtain a calcium carbonate material comprising GCC and PCC, with aspecific steepness factor (defined as d₃₀/d₇₀×100, where d_(x) is theequivalent spherical diameter relative to which x % by weight of theparticles are finer) of at least about 30, preferably of at least about40, and most preferably of at least about 45. Such a material leads tosuperior properties of the paper coated with such a material, notably interms of gloss.

It is also an object of the present invention to provide a process toobtain a calcium carbonate material comprising GCC and PCC, with asteepness factor of at least about 30, preferably of at least about 40,and most preferably of at least about 45, in a cost efficient manner,wherein GCC and PCC are co-ground, possibly with at least anothermineral material.

Another object of the present invention lies in the co-ground calciumcarbonate material (i.e.: aqueous mineral slurries containing theco-ground GCC and PCC and dry products containing the co-ground GCC andPCC) obtained through this process.

Another object of the present invention lies in the uses of suchproducts in any sector making use of mineral materials, and notably inthe paper, paint and plastic industries.

Many types of mineral are used in the paper coating formulations for thepaper industry. Clay has traditionally been used for this purpose due toits low cost relative to other mineral pigments.

Calcium carbonate (CaCO₃) is used as both a coating and filling pigment,and is notably known to improve some of the optical properties of thefinal product, such as gloss, opacity or brightness. Calcium carbonatecan be of two types: ground or natural calcium carbonate referred to asGCC, and synthetic or precipitated calcium carbonate referred to as PCC.

Ground calcium carbonate is calcium carbonate obtained from naturalsources, such as limestone, marble or chalk, and processed through atreatment such as grinding. Precipitated calcium carbonate is asynthesized material, generally obtained by precipitation followingreaction of carbon dioxide and lime in an aqueous environment. This PCCmay be rhombohedral and/or scalenohedral and/or aragonitic. According tothe needs of the man skilled in the art, this GCC or PCC mayadditionally be surface treated, for example with stearine

For many years, there has been need to supply the skilled man in the artwith mineral slurries comprising GCC and PCC, since it may be ofinterest that both be present in the paper coating formulations, inorder to regulate more precisely the final properties of the coatedpaper. Publications referring to the use of both natural andprecipitated calcium carbonate in the paper industry include, forinstance, “PCC or GCC, factors determining calcium carbonate choice inalkaline conversion” (published following the November 1995 28^(th) Pulpand Paper Annual Meeting) and “GCC vs. PCC as the primary filler foruncoated and coated wood-free paper” (Tappi Journal 2000, 83(5), pp 76):these publications refer to the properties of PCC/GCC blends for use inpaper industry. In “Chalk: a calcium carbonate for the high-filledsheet” (TAPPI Proceedings, Apr. 5-8 1992, Papermakers Conference, Book2, Opryland Hotel, Nashville Tenn., TAPPI Press, pp. 515-520), theauthor suggests that drawbacks associated with PCC may be overcome byusing this mineral in conjunction with other fillers, such as GCC.Finally, in “Coating structure with calcium carbonate pigments and itsinfluence on paper and print gloss” (Pulp & Paper Canada, 2004, 105(9),pp. 43-46), the influence of different pigment blends including GCC andPCC on paper properties including gloss and print gloss is investigated.The Applicant underlines that these publications appear to belong to thetechnical background of the invention, since they attest to the need toobtain mixtures of GCC and PCC for use in paper industry. However, noneof these publications teach or reveal the co-grinding of both GCC andPCC, and the further possibility to obtain a co-ground product with aspecific steepness factor, which is one of the objects of the presentinvention.

With further reference to the need of the skilled man in the art toimprove some of the final properties of the coated paper, there is alsoan additional need for the skilled man to improve some of the opticalproperties of the final product, such as gloss. Faced with thisrequirement, the skilled man in the art knows that the “steepnessfactor” of the mineral material used in the paper coating formulation isa criteria of main importance: the selection of specific values for thesteepness factor, in relation with the choice of a specific mineralmaterial, can lead to an improvement in the optical properties of thecoated paper. At this point, the Applicant indicates that a common wayto define the steepness factor is “the ratio of the d_(x) equivalentspherical diameter (at which x % by weight of the particles are finer)to the d_(y) equivalent spherical diameter (at which y % by weight ofthe particles are finer), multiplied by 100. In such a way, for a givenmineral material in slurry form or in the form of a dry powder, thesteepness factor can be regarded the steepness of the correspondinggranulometric curve.

In this area, the skilled man in the art knows WO 2003/089 524, whichtargets a high degree of brightness, whiteness, and fluorescence inpaper coatings or filler compositions used in cellulose based substrateand light weight coating base paper. The proposed solution consists of ahydrous kaolin having a GE brightness of at least 90 and a steepnessfactor (d₃₀/d₇₀×100) of at least about 39. It has to be noted that thisdocument does not offer teachings relating to calcium carbonate, sinceone of the requirements of the inventors is precisely to avoid the useof this mineral.

The prior art also reveals some documents dealing with the use ofcalcium carbonate of a single-type, in conjunction with specificsteepness factors. EP 0 894 836 discloses a slurry consisting of water,commercially available dispersant which prevents the dissociation ofagglomerated pigment in the slurry, and agglomeratedcarbonate-containing pigment with a particle size distribution in which80-99% by weight are below 2 μm in size, 50-90% by weight are below 1 μmand 0-10% by weight are below 0.2 μm, a steepness factor (ratio ofdiameter at 50% by weight to diameter at 20% by weight) is of 1.5-2.0and a porosity is of 45-65%. It is clearly that this invention dealssolely with natural calcium carbonate of the calcite, marble andchalk-type; moreover, the invention lies in a dispersing process anddoes not teach the grinding of the abovesaid carbonate-containingpigment. US 2002 155 055 addresses the problem of reducing the width ofparticle size distribution of calcium carbonate compositions for use inpaper, but is exclusively focused on ground calcium carbonate, asrecognized by the inventors (see [0007]). The proposed solution lies ina process comprising the step of forming a dispersant-free aqueoussuspension of natural calcium carbonate, wet-grinding the suspension toproduce a calcium carbonate composition having steepness ratio (A) andaging the suspension at temperature below 35° C. to produce a calciumcarbonate composition having steepness ratio (B) smaller than the ratio(A). In this document, the steepness factor is defined as the averagediameter of the particles in the 75% mass divided by the averagediameter of the particles in the 25% mass, when the size distribution isrepresented using a Sedigraph™.

There are also prior art documents which deal with the use of calciumcarbonate of a single type or of both types (GCC and PCC blends), inconjunction with at least one other mineral matter (and notably kaolin),and disclosing some particular values for the steepness factor of eachmaterial and/or of the final blend. WO 2003/093 577 teaches that, inorder to improve gloss, opacity, brightness and smoothness of paper,specific particulate pigments may be useful in the paper coatingformulations. These pigments comprise a first component, which is PCCand a second component which is a processed particulate hydrous kaolinclay having a shape factor of at least 25 and a steepness of at least20, or a first component, which is a PCC having a spherical particleshape and a second component which is a processed particulate hydrouskaolin clay having a shape factor at least 45, and a mean equivalentparticle diameter of less than 0.5 or a first component which is a PCCand a second component, which is a processed particulate hydrous kaolinclay having a shape factor less than 25. Moreover, WO 2002/016 509teaches that to improve the optical properties of paper and printabilityof paper coatings, it is advantageous to use kaolin having a meanparticle size of 0.7-3 μm and a shape factor of at least 60; this typeof kaolin can be used in combination with another filler such as talc,calcium sulfate and/or alkaline earth metal carbonate. Finally, WO2000/066 510 teaches that pigment compositions comprising a blend of afine kaolin produced from block kaolin clay, and a calcium carbonatewhich may be either GCC or PCC, wherein both particles have a medianparticle size less than 0.8 μm, and a steepness factor, defined as100×d₃₀/d₇₀, greater than 38, and wherein the kaolin/carbonate weightratio is of 40/60, preferably of 50/50, can improve the opticalproperties and printability of coated paper. While the latter threedocuments refer to the use of blends of calcium carbonate (possibly ofboth the GCC and PCC type), and necessarily kaolin, which is not arequirement of the present invention, they do not teach nor reveal thepossibility to co-grind PCC and GCC, or even to the possibility ofco-grinding kaolin with at least one type of calcium carbonate mineral.

Closer from the scope of the present invention, there are also documentsdealing with the use of mixtures of GCC and PCC, notably for use inpaper formulations to enhance some of the optical properties of thecoated paper. DE 4 128 570 discloses a carbonate filler and pigment withspecified particle shape and size for filling and coating paper, givinghigh opacity, degree of whiteness and filler content. Such carbonatefiller and pigment have a rhombohedral or round particle shape, agradient factor (ratio of particle diameter in μm at 50/20 weight %) of1.1-1.4, a ratio R of % particles finer than 1 μm/% particles finer than0.6 μm ranging from 8 to 19, and average statistic particle diameterranging from 0.4 to 1.5 μm. Finally, WO 2004/059 079 disclosesparticulate pigment compositions, useful in paper, comprising a firstpigment which is ground calcium carbonate and a second pigment which isprecipitated or ground calcium carbonate, carbonate, the first andsecond pigments having different size distribution steepness factors(100×d₃₀/d₇₀). More precisely, the claimed particulate pigmentcomposition comprises two pigment components. The first comprisesparticulate GCC carbonate having a steepness factor of 30-45, and thesecond comprises PCC with a steepness factor of 55-75, and diameter ofat most 0.5 μm, or GCC with a steepness factor of 40-55.

First, it must be noted that neither of these two documents reveals aspecific steepness factor of the final product, which is one of theobjects of the present invention. Secondly, none of these documentsteaches the co-grinding of GCC and PCC. It appears clearly that theseinventions are based on the mixing of both GCC and PCC calcium carbonatetypes: the skilled man encounters new problems. Finely ground PCC with acertain granulometry is commonly desired, said granulometry beingachieved by grinding in dry and/or aqueous media. Nevertheless, afterthis grinding step, it has been observed that the resulting fine PCCparticles collapse and must subsequently be de-agglomerated (processesin order to de-agglomerate such finely ground PCC are notably disclosedin JP 2001 089 505, JP 56 104 713, U.S. Pat. No. 6,143,065 or U.S. Pat.No. 5,279,663) by mechanical means and/or by the addition ofde-agglomerating agents: this addition step represents an additionalexpense in the PCC production process; there is a need to perform thisde-agglomeration step in a cost-efficient manner. Finally, whenco-grinding GCC and PCC as opposed to separately grinding each componentprior to blending, in particular when using the specificceria-containing beads described hereafter, a surprising increase ingrinding efficiency (decrease in total specific energy required toobtain the final products with the desired steepness factor) has beenobserved.

As indicated above, there is a need to provide the skilled man in theart with mineral slurries comprising both GCC and PCC for use in papermanufacturing, the steepness factor of the corresponding mineral beingselected so that optical properties of the coated paper are improved,and in a cost efficient way in order to avoid, notably, the additionalcostly step of de-agglomerating PCC as necessary in the case of simplymixing of GCC and PCC.

Via the present invention, a new process of manufacturing a calciumcarbonate mineral material comprising both GCC and PCC, without thedrawbacks present in the prior art, has surprisingly been found. Thisprocess lies in a process for the preparation of co-ground calciumcarbonate material of the GCC and PCC type, presenting a steepnessfactor of at least about 30%, preferably of at least about 40%, and mostpreferably of at least about 45%, comprising the step of co-grinding GCCand PCC, possibly with at least another mineral material.

More precisely, the invention lies in a process of manufacturing aco-ground calcium carbonate material of the GCC and PCC type, presentinga steepness factor of at least about 30, preferably of at least about40, and most preferably of at least about 45, and characterised in thatit comprises the steps of:

-   -   (a) providing at least one calcium carbonate material,        optionally in the form of an aqueous suspension,    -   (b) co-grinding GCC and PCC, optionally with at least another        mineral material,    -   (c) optionally screening and/or upconcentrating the co-ground        calcium carbonate material obtained following step (b),    -   (d) optionally drying the co-ground calcium carbonate material        obtained following step (b) or (c)

This process allows the skilled man in the art to obtain co-groundcalcium carbonate material in the form of an aqueous suspension and/orin the form of a dry product comprising both GCC and PCC, which maynotably be used in the paper industry. Moreover, and due to the specificselection concerning the steepness factor of the final product, whichmust be of at least about 30, preferably of at least about 40, and mostpreferably of at least about 45, high gloss properties are achieved inthe coated paper.

Lastly, it has surprisingly been found that following the co-grindingstep, significant additional PCC de-agglomeration is no longernecessary: as such, the process according to the invention is lessexpensive than processes of the prior art based on the simple mixing ofboth GCC and PCC, which requires a first de-agglomeration of PCC.Finally, when co-grinding GCC and PCC as opposed to separately grindingeach component prior to blending, in particular when using the specificceria-containing beads described hereafter, a surprising increase ingrinding efficiency (decrease in total specific energy required toobtain the final products with the desired steepness factor) has beenobserved.

The Applicant would like also to mention EP 0 850 880, which disclosesan aqueous slurry or dehydrated wet cake with a 25-75% solidsconcentration comprising a mixture of PCC and a viscosity reducing agentwhich is dispersed in a mixer to give a slurry with a viscosity below1000 cp (at 25° C.), and which comprises 0.2-3 μm median diametercalcium carbonate particles. The slurry is then admixed with 1.5-30 μmmedian diameter dry ground calcium carbonate particles to give a weightratio of to (II) of 20:80 to 80:20 and a solids concentration of 60-85%.The slurry is next dispersed in a mixer to a viscosity below 1000 cp andfinally dispersed in a sand grinding mill to give a product aqueousslurry comprising 0.2-2 μm median diameter calcium carbonate particles.The EP 0 850 880 patentee teaches the above process as a solution tocounter the high shear rheology difficulties encountered when the GCCcomponent is wet ground, which is a technical problem different than theone solved by the present invention. By contrast, in the presentinvention, it has firstly been found that a wet grinding is alsoacceptable without a loss of gloss. Furthermore, the patentee makes noreference to any gain in production process energy via this process thatnecessitates a dry grinding of GCC. Finally, this patent does not teachthat a desirable steepness factor can be reached for gloss improvementby an energetically economic process.

A first object of the invention consists in a process of manufacturing aco-ground calcium carbonate material comprising GCC and PCC, presentinga steepness factor of at least about 30, preferably of at least about40, and most preferably of at least about 45, and characterised in thatit comprises the steps of:

-   -   (a) providing at least one calcium carbonate material,        optionally in the form of an aqueous suspension,    -   (b) co-grinding GCC and PCC, optionally with at least another        mineral material,    -   (c) optionally screening and/or upconcentrating the co-ground        calcium carbonate material obtained following step (b),    -   (d) optionally drying the co-ground calcium carbonate material        obtained following step (b) or (c)

The process according to the invention is characterised in that in step(a), the calcium carbonate material is provided as an aqueoussuspension, and in that this aqueous suspension contains from 20% to 80%by dry weight of calcium carbonate, preferably from 50% to 75%, and mostpreferably from 50% to 70%. Said aqueous suspension may result from thedispersion of calcium carbonate material in the form of a wet cake

According to this specific embodiment, the process according to theinvention is also characterised in that the calcium carbonate materialprovided in the form of an aqueous suspension is GCC.

In this particular embodiment, the wet ground natural calcium carbonatemay be subjected to a wet benefication step prior to step (b), allowingthe removal of impurities, such as silicate impurities, for instance byfroth flotation.

In another embodiment, the process according to the invention is alsocharacterised in that step (c) is carried out.

In another embodiment, the process according to the invention is alsocharacterised in that step (d) is carried out.

More generally, the process according to the invention is alsocharacterised in that the co-grinding of GCC and PCC during step (b) isconducted in aqueous medium, wherein the concentration of calciumcarbonate ranges from 20% to 80% (by dry weight of calcium carbonate),preferably from 50% to 75%, and most preferably from 50% to 70%.

The process according to the invention is also characterised in that atleast one dispersing and/or grinding aid agent present in a weight %relative to the total dry mineral material ranging from 0 to 2%,preferably from 0.2% to 1.4%, and most preferably from 0.5% to 1.2% maybe added before, during or after co-grinding in step b).

The skilled man in the art will choose the dispersing and/or grindingaid agent as a function of the properties he wishes to achieve. He canuse, for instance, homopolymers of (meth)acrylic acid and/or copolymersof (meth)acrylic acid in combination with other water soluble monomers,such homo- and copolymers, which are totally or partially neutralised.

Such dispersants may be added to obtain a stable Brookfield™ viscosityof less than 3000 mPa·s, preferably of less than 1000 mPa·s measured at25° C.

The process according to the invention is also characterised in that theco-grinding of GCC and PCC during step (b) is conducted in the presenceof at least another mineral material selected from among talc, clay,Al₂O₃, TiO₂ or mixtures thereof.

More preferably, the other mineral material is selected among from talc,clay or mixtures thereof.

Most preferably, the other mineral material is talc or clay.

The process according to the invention is also characterised in that theco-grinding of GCC and PCC during step (b) occurs at a pH of above 7.

In another embodiment, the process according to the invention is alsocharacterised in that the co-grinding of GCC and PCC during step (b)occurs at a pH of above 10.

In another embodiment, the process according to the invention is alsocharacterised in that the co-grinding of GCC and PCC during step (b)occurs at a pH of above 11.

This pH increase can be the result of, for example, one or more of thefollowing: by the addition of a base, preferably of a base of a mono ordivalent cation, most preferably of sodium or calcium, by the additionof an alkaline preparation of a biocide, or by the release of hydroxide,such a Ca(OH)₂, during grinding of a material, such as during theco-grinding of PCC and GCC. The Applicant indicates that he knows ofFrench patent application number 05 00779, not yet published at the dateof filing of the present patent application, which mentions biocidesthat may be added during the grinding step (b).

The process according to the invention is also characterised in that thegrinder contents are subject to a temperature rise, to above 60° C.,preferably to above 90° C., and most preferably to above 100° C., duringstep (b).

This temperature refers to the temperature reached by the mill contentsat any one point in the mill. In particular, the mill contents at themill base may be subject to a higher temperature as a result of a higherhydrostatic pressure.

The process according to the invention is also characterised in that thePCC present when co-grinding during step (b) accounts for 10% to 90% ofthe total combined PCC and GCC weight, preferably from 20% to 80% of thetotal combined PCC and GCC weight, and most preferably from 30% to 70%of the total combined PCC and GCC weight.

The process according to the invention is also characterised in that theco-grinding of GCC and PCC during step (b), is conducted in the presenceof ceria-containing zirconium oxide grinding beads as grinding media,such beads having:

-   -   a ceria content of between 14% and 20% by weight relative to the        total weight of said bead, preferably of between 15% and 18% by        weight relative to the total weight of said bead, and most        preferably of approximately 16% by weight relative to the total        weight of said bead; and    -   an average grain size after sintering of the grains forming said        beads of less than 1 μm, preferably of less than 0.5 μm, and        most preferably of less than 0.3 μm.

This grain size is determined by analysis of scanning electronmicroscope images of the beads. Bead ceria content is analysed by ICPOptical Emission Spectrometry

The process according to the invention is also characterised in that thebeads have an original diameter prior to grinding of between 0.2 mm and1.5 mm, preferably of between 0.4 mm and 1.0 mm.

Another object of the present invention lies in the co-ground calciumcarbonate material comprising GCC and PCC, characterised in that it isobtained by the process according to the invention.

Another object of the present invention lies in calcium carbonatematerial comprising GCC and PCC, characterised in that it is in the formof an aqueous suspension, and presents a steepness factor of at leastabout 30, preferably of at least about 40, and most preferably of atleast about 45.

According to the above embodiment, the co-ground calcium carbonatematerial in the form of an aqueous suspension is also characterised inthat it contains from 20% to 80% by dry weight of calcium carbonatematerial, preferably from 40% to 75% by dry weight of calcium carbonatematerial, and most preferably from 60% to 70% by dry weight of calciumcarbonate material.

The co-ground calcium carbonate material in the form of an aqueoussuspension is also characterised in that the PCC present accounts for10% to 90% of the total combined PCC and GCC weight, preferably from 20%to 80% of the total combined PCC and GCC weight, and most preferablyfrom 30% to 70% of the total combined PCC and GCC weight.

The co-ground calcium carbonate material in the form of an aqueoussuspension is also characterised in that it features a d₅₀ from about0.2 μm to 2.0 μm, preferably from 0.2 μm to 0.8 μm, and most preferablyfrom 0.25 μm to 0.45 μm. This d₅₀ is measured using a Sedigraph™ 5100.

The co-ground calcium carbonate material in the form of an aqueoussuspension is also characterised in that the aqueous suspension containsat least one dispersing and/or grinding aid agent, such dispersingand/or grinding aid agent being present in a weight % relative to thetotal dry mineral material ranging from 0 to 2%, preferably from 0.2% to1.4%, most preferably from 0.5% to 1.2%.

The co-ground calcium carbonate material in the form of an aqueoussuspension is also characterised in that the slurry water passed througha 40 μm sieve contains less than 1000 ppm of ZrO₂ and less than 200 ppmCeO₂. ZrO2 and CeO2 contents are determined by ICP-OES.

The co-ground calcium carbonate material in the form of an aqueoussuspension is also characterised in that the slurry water features aZrO₂/CeO₂ weight ratio of from 4 to 6.5, preferably of from 4.6 to 5.7,and most preferably of 5.3.

The co-ground calcium carbonate material in the form of an aqueoussuspension is also characterised in that it contains:

-   -   a fraction of particles finer than 1 μm of greater than 80%,        preferably of greater than 85%, more preferably of greater than        90%, and even more preferably of greater than 95%, and    -   a BET specific surface area of less than 25 m²/g.

When the fraction of particles finer than 1 μm is greater than 95%, theBET specific surface area is preferably less than 25 m²/g. When thefraction of particles finer than 1 μm is greater than 90%, greater than85%, and greater than 80%, the BET specific surface area is preferablyless than 20 m²/g, less than 18 m²/g, and less than 15 m²/g,respectively.

Another object of the present invention lies in the co-ground calciumcarbonate material comprising GCC and PCC, characterised in that it isin the form of a dry product, presenting a steepness factor of at leastabout 30, preferably of at least about 40, and most preferably of atleast about 45.

The co-ground calcium carbonate material in the form of a dry product isalso characterised in that the PCC present accounts for 10% to 90% ofthe total combined PCC and GCC weight, preferably from 20% to 80% of thetotal combined PCC and GCC weight, and most preferably from 30% to 70%of the total combined PCC and GCC weight.

The co-ground calcium carbonate material in the form of a dry product isalso characterised in that it contains:

-   -   a fraction of particles finer than 1 μm of greater than 80%,        preferably of greater than 85%, more preferably of greater than        90%, and even more preferably of greater than 95%, and    -   a BET specific surface area of less than 25 m²/g.

When the fraction of particles finer than 1 μm is greater than 95%, theBET specific surface area is preferably less than 25 m²/g. When thefraction of particles finer than 1 μm is greater than 90%, greater than85%, and greater than 80%, the BET specific surface area is preferablyless than 20 m²/g, less than 18 m²/g, and less than 15 m²/g,respectively.

The co-ground calcium carbonate material in the form of a dry product isalso characterised in that it features a d₅₀ from about 0.2 μm to 2.0μm, preferably from 0.2 μm to 0.8 μm, and most preferably from 0.25 μmto 0.45 μm. This d₅₀ is measured using a Sedigraph™ 5100.

The co-ground calcium carbonate material in the form of a dry product isalso characterised in that it features a ZrO₂/CeO₂ weight ratio of from4 to 6.5, preferably of from 4.6 to 5.7, and most preferably of 5.3.

Finally, another object of the present invention lies in the use of theco-ground calcium carbonate material according to the invention, in anysector making use of mineral material, and notably in the paper, paintand plastic industries.

EXAMPLES

The following examples are intended to illustrate certain embodiments ofthe invention and are non-limitative.

Median diameter (d₅₀) and the fraction of particles featuring a diameterbelow a given diameter value were measured using a Sedigraph™ 5100.

Example 1 Comparative Example

Ground calcium carbonate presenting a median diameter of 1.5 μm waswet-ground at a solids content of 74.5% in the presence of the followingadditives: 1.51% sodium polyacrylate, in a two-pass process usingceria-comprising zirconium oxide grinding beads featuring a median beaddiameter of 0.45 mm, a CeO₂ content of 1.6% by weight relative to thetotal bead weight, and a grain size after sintering of 0.4 μm(determined by evaluations of SEM images). The specific grinding energyrequired to obtain a final GCC with a steepness factor of about 35% wasof 270 kWh/t.

The obtained slurry of the ground GCC material featuring a subsequentlydiluted solids content of 75% was then added to a standard paper coatingformulation made up of the following proportions of components:

 100 parts ground GCC material 10.5 parts SBR latex  0.5 parts syntheticthickener  0.2 parts polyvinyl alcohol  0.2 parts optical brighteningagent

The above coating was adjusted to a final solids content of 68% andapplied on a standard pre-coated wood-free base paper with a grammage of71 g/m² at a coat weight of 10 g/m²/side. This coated base paper wasthen calendered using a supercalender under the following calenderingconditions: calender speed of 800 in/min, calender load of 200 kN/cm anda temperature of 105° C.

The gloss of the coated paper surface was of 70% Tappi 75°.

Example 2 Illustration of the Process According to the Invention

A 76% solids content slurry of ground calcium carbonate presenting amedian GCC diameter of 1.4 μm was ground in the presence of a 51% solidscontent PCC slurry with a median PCC diameter of 0.75 μm. The PCC to GCCweight ratio in the mill was of 50:50. The total solids content of theslurry in the mill was of 61% and a median diameter of 1.1. The grindercontents were then co-ground in the presence of the following totaladditives content: 0.95 weight % sodium polyacrylate, usingceria-comprising zirconium oxide grinding beads featuring a median beaddiameter of 0.45 mm, a CeO2 content of 16% by weight relative to thetotal bead weight, and a grain size after sintering of 0.4 μm(determined by evaluations of SEM images). The specific grinding energyrequired to obtain a final co-ground product with a steepness factor ofabout % 42 was of 200 kWh/t.

The obtained slurry of the co-processed material featuring a solidscontent of 70.2% was then added to a standard paper coating formulationmade up of the following weight proportions of components:

 100 parts co-processed material 10.5 parts SBR latex  0.5 partssynthetic thickener  0.2 parts polyvinyl alcohol  0.2 parts opticalbrightening agent

The above coating was adjusted to a final solids content of 68% andapplied on a standard pre-coated wood-free base paper with a grammage of71 g/m² at a coat weight of 10 g/m²/side. This coated base paper wasthen calendered using a supercalender under the following calenderingconditions: calender speed of 800 m/min, calender load of 200 kN/cm anda temperature of 105° C.

The gloss of the coated paper surface was of 72% Tappi 75°.

The above results are summarised in Table 1.

TABLE 1 Example 1 Example 2 Fraction of particles finer than 1   97%  97% μm in the final ground product BET specific surface area of the 28g/m² 23 g/m² final ground product Steepness factor of the final 35 42ground product Median diameter of the final 0.27 μm 0.27 μm groundproduct Total specific grinding energy 270 kWh/t 200 kWh/t required toproduce the product Tappi gloss of paper coated with   70%   72% aformulation comprising the product Brightness of paper coated with 95.1%96.5% a formulation comprising the product Opacity of paper coated witha 89.7% 90.2% formulation comprising the product

Table 1 illustrates that the process according to the invention requiresless specific grinding energy, and leads to an equal/improved gloss,relative to a process of the prior art.

Example 3 Comparative Example

This example illustrates a blend of PCC and GCC, in which each componentis first separately ground prior to being mixed.

A 48% solids aqueous slurry of PCC starting material having thecharacteristics indicated under Example 3 in Table 2 was ground in amedia mill using yttrium-stabilised zirconium silicate grinding beadsfeaturing a bead diameter prior to grinding of 0.6 to 1.0 mm. A total of50 kWh/t specific grinding energy Was expended in order to obtain a PCCend material having the end material characteristics indicated in Table2. The final solids content of this subsequently upconcentrated PCCslurry was of 68%.

Separately, a 74% solids aqueous slurry of GCC starting material havingthe characteristics indicated under Example 3 in Table 2 was ground in amedia mill using yttrium-stabilised zirconium silicate grinding beadsfeaturing a bead diameter prior to grinding of 0.6 to 1.0 mm. A total of210 kWh/t specific grinding energy was expended in order to obtain a GCCend material having the end material characteristics indicated in Table2. The final solids content of this GCC slurry was of 75%.

The PCC and GCC slurries were then mixed so as to obtain a PCC/GCC blendmaterial with a PCC:GCC weight ratio of 30:70. This slurry was thenadded to a standard paper coating formulation made up of the followingweight proportions of components:

 100 parts PCC/GCC blend material 10.5 parts SBR latex  0.5 partssynthetic thickener  0.2 parts polyvinyl alcohol  0.2 parts opticalbrightening agent

The above coating was adjusted to a final solids content of 68% andapplied on a standard pre-coated wood-free base paper with a grammage of71 g/m² at a coat weight of 10 g/m²/side. This coated base paper wasthen calendered using a supercalender under the following calenderingconditions: calender speed of 800 m/min, calender load of 200 kN/cm anda temperature of 105° C.

The optical properties of the coated paper surface are given in Table 2.

Example 4 Example According to the Invention

This example illustrates a co-ground PCC and GCC obtained by a processaccording to the invention.

A 74% solids content slurry of ground calcium carbonate presenting thecharacteristics listed under Example 4 in Table 2 was ground in thepresence of a 48% solids content PCC slurry with the characteristicslisted under Example 4 in Table 2 in a media mill. The PCC to GCC weightratio in the mill was of 30:70 and the solids content of 65.9%. Thegrinder contents were co-ground using yttrium-stabilised zirconiumsilicate grinding beads featuring a bead diameter prior to grinding of0.6 to 1.0 mm. A total of 116 kWh/t specific grinding energy wasexpended in order to obtain a GCC/PCC co-ground end material having theend material characteristics indicated in Table 2. The final solidscontent of this GCC slurry was of 70.3%.

This slurry was then added to a standard paper coating formulation madeup of the following weight proportions of components:

 100 parts PCC/GCC co-ground material 10.5 parts SBR latex  0.5 partssynthetic thickener  0.2 parts polyvinyl alcohol  0.2 parts opticalbrightening agent

The above coating was adjusted to a final solids content of 68% andapplied on a standard pre-coated wood-free base paper with a grammage of71 g/m² at a coat weight of 10 g/m²/side. This coated base paper wasthen calendered using a supercalender under the following calenderingconditions: calender speed of 800 m/min, calender load of 200 kN/cm anda temperature of 105° C.

The optical properties of the coated paper surface are given in Table 2.

TABLE 2 Example 3: Example 4: PCC/GCC PCC/GCC Blend Co-Ground ProductName Material Material Starting material characteristics GCC d₅₀ (μm)1.4 1.4 Steepness factor 28 28 PCC d₅₀ (μm) 0.75 0.75 Steepness factor55 55 End material characteristics GCC d₅₀ (μm) 0.40 — Steepness factor34 — PCC d₅₀ (μm) 0.38 — Steepness factor 40 — PCC/GCC PCC/GCC weightratio 30/70 30/70 d₅₀ (μm) 0.38 0.40 Steepness factor 37 38 Fraction ofparticles 89.5 88.8 with a diameter less than 2 μm (%) BET specificsurface 18.1 18.2 area (g/m²) Total specific grinding energy 162 kWh/t116 kWh/t Characteristics of paper coated with the end material Papergloss (Tappi 75°) 70.5%   72% Opacity 90.4% 90.5% Brightness R457 97.9%97.9%

Table 2 indicates that the process to prepare a co-ground PCC/GCCmaterial according to the invention requires less grinding energy ascompared to that required to prepare a comparable blend of PCC and GCC,without any loss or with an improvement in optical properties.

Example 5

This example illustrates the use of a process according to the inventionwherein 3 minerals, a natural calcium carbonate a precipitated calciumcarbonate and a clay, are co-ground with the use of ceria-containingzirconium oxide grinding beads with a ceria content of 16% by weightrelative to the total weight of said bead, an average grain size aftersintering of the grains forming said bead of 0.4 μm, and a median beaddiameter of 0.45 mm. The co-ground material is then added to a coatingformulation used to coat a base paper, and the resulting gloss ismeasured.

The following materials were co-ground:

-   -   a 74% solids content slurry of ground calcium carbonate        presenting a median GCC diameter of 1.4 μm and prepared using        0.27% weight (by weight of dry GCC) of an homopolymer of acrylic        acid,    -   a 51% solids content PCC slurry with a median PCC diameter of        0.8 μm and prepared using 0.7% weight (by dry weight of PCC) of        an homopolymer of acrylic acid,    -   and a 68% solids content slurry of clay commercialized by HUBER™        under the name Lithoprint™.

The weight ratio PCC:GCC:clay in the mill was of 45:45:10.

The total solids content of the slurry in the mill was of 72% and themedian diameter was of 0.4 and 0.5 μm for the 2 tests illustrating theinvention.

The grinder contents were then co-ground in the presence of thefollowing total additives content:

-   -   respectively 0.4 and 0.2 weight % (by dry weight of mineral        matter) of an homopolymer of acrylic acid, where 14 mol % of the        carboxylic functions are neutralized by sodium hydroxyde, having        a molecular weight of 5 600 g/mol, and a polydispersity equal to        2.4,    -   using ceria-comprising zirconium oxide grinding beads featuring        a median bead diameter of 0.45 mm, a CeO2 content of 16% by        weight relative to the total bead weight, and a grain size after        sintering of 0.45 μm,        leading to a coground material exhibiting a median diameter        respectively of 0.4 and 0.5 μm.

The 2 obtained slurry of the co-processed material was then added to astandard paper coating formulation made up of the following weightproportions of components:

100 parts co-processed material  11 parts SBR latex (DL 966commercialized by DOW CHEMICALS ™)  0.5 parts synthetic thickener (CMCFF5 commercialized by FINNFIX ™)  0.4 parts polyvinyl alcohol (PVA 4-98commercialized by CLARIANT ™)  0.6 parts optical brightening agent(Blancophor ™ P commercialized by BAYER ™)

The above coating was applied on a standard topcoat base paper with agrammage of 78 g/m² at a coat weight of 10 g/m²/side. This coated basepaper was then calendered using a supercalender under the followingcalendering conditions: calender speed of 300 m/min, calender load of170 kN/m and a temperature of 80° C.

For the coground material exhibiting a median diameter of 0.4 μm, thegloss of the coated paper surface was of 73% Tappi 75° and 45% DIN 75°.

By comparison, the same coating manufactured with 100 parts of a GCChaving a median diameter of 0.4 μm was of 70% Tappi 75° and 35% DIN 75°.

For the coground material exhibiting a median diameter of 0.5 μm, thegloss of the coated paper surface was of 68% Tappi 75° and 40% DIN 75°.

By comparison, the same coating manufactured with 100 parts of a GCChaving a median diameter of 0.4 μm was of 63% Tappi 75° and 33% DIN 75°.

1-18. (canceled)
 19. Co-ground calcium carbonate material comprising GCCand PCC, obtained by the process of co-grinding GCC and PCC to obtainco-ground calcium carbonate material having a steepness factor of atleast 30, preferably of at least 40, and most preferably of at least 45.20. Co-ground calcium carbonate material comprising GCC and PCC, in theform of an aqueous suspension having a steepness factor of at least 30,preferably of at least 40, and most preferably of at least
 45. 21.Co-ground calcium carbonate material according to claim 20, comprisingfrom 20 to 80% by dry weight of calcium carbonate material, preferablyfrom 40 to 75% by dry weight of calcium carbonate material, and mostpreferably from 60 to 70% by dry weight of calcium carbonate material.22. Co-ground calcium carbonate material according to claim 20, whereinthe PCC present accounts for 10% to 90% of the total combined PCC andGCC weight, preferably from 20% to 80% of the total combined PCC and 0CCweight, and most preferably from 30% to 70% of the total combined PCCand GCC weight.
 23. Co-ground calcium carbonate material according toclaim 20, having a d₅₀ from about 0.2 μm to 2 μm, preferably from 0.2 μmto 0.8 μm, and most preferably from 0.25 μm to 0.45 μm.
 24. Co-groundcalcium carbonate material according to claim 20, wherein the aqueoussuspension contains at least one dispersing and/or grinding aid agent,such dispersing and/or grinding aid agent being present in a weight %relative to the total dry mineral material ranging from 0 to 2%,preferably from 0.2 to 1.4%, most preferably from 0.5% to 1.2%. 25.Co-ground calcium carbonate material according to claim 20, whereinslurry water from the aqueous suspension passed through a 40 μm sievecontains less than 1000 ppm of ZrO₂ and less than 200 ppm CeO₂. 26.Co-ground calcium carbonate material according to claim 20, whereinslurry water from the aqueous suspension comprises a ZrO₂/CeO₂ weightratio of from 4 to 6.5, preferably of from 4.6 to 5.7, and mostpreferably of 5.3.
 27. Co-ground calcium carbonate material according toclaim 20, comprising: a fraction of particles finer than 1 μm of greaterthan 80%, preferably of greater than 85%, more preferably of greaterthan 90%, and even more preferably of greater than 95%, and a BETspecific surface area of less than 25 m²/g.
 28. Co-ground calciumcarbonate material according to claim 27, wherein for a fraction ofparticles finer than 1 μm of greater than 95%, the BET specific surfacearea is less than 25 m²/g.
 29. Co-ground calcium carbonate materialaccording to claim 27, wherein for a fraction of particles finer than 1μm of greater than 90%, the BET specific surface area is less than 20m²/g.
 30. Co-ground calcium carbonate material according to claim 27,wherein for a fraction of particles finer than 1 μm of greater than 85%,the BET specific surface area is less than 18 m²/g.
 31. Co-groundcalcium carbonate material according to claim 27, wherein for a fractionof particles finer than 1 μm of greater than 80%, the BET specificsurface area is less than 15 m²/g.
 32. Co-ground calcium carbonatematerial comprising GCC and PCC, in the form of a dry product, having asteepness factor of at least 30, preferably of at least 40, and mostpreferably of at least
 45. 33. Co-ground calcium carbonate materialaccording to claim 32, wherein the PCC present accounts for 10% to 90%of the total combined PCC and GCC weight, preferably from 20% to 80% ofthe total combined PCC and GCC weight, and most preferably from 30% to70% of the total combined PCC and 0CC weight.
 34. Co-ground calciumcarbonate material according to claim 32, comprising: a fraction ofparticles finer than 1 pm of greater than 80%, preferably of greaterthan 85%, more preferably of greater than 90%, and even more preferablyof greater than 95%, and a BET specific surface area of less than 25m²/g.
 35. Co-ground calcium carbonate material according to claim 34,wherein for a fraction of particles finer than 1 μm of greater than 95%,the BET specific surface area is less than 25 m²/g.
 36. Co-groundcalcium carbonate material according to claim 34, wherein for a fractionof particles finer than 1 μm of greater than 90%, the BET specificsurface area is less than 20 m²/g.
 37. Co-ground calcium carbonatematerial according to claim 34, wherein for a fraction of particlesfiner than 1 μm of greater than 85%, the BET specific surface area isless than 18 m²/g.
 38. Co-ground calcium carbonate material according toclaim 34, wherein for a fraction of particles finer than 1 μm of greaterthan 80%, the BET specific surface area is less than 15 m²/g. 39.Co-ground calcium carbonate material according to claim 32, having a d₅₀from about 0.2 μm to 2 μm, preferably from 0.2 μm to 0.8 μm, and mostpreferably from 0.25 μm to 0.45 μm.
 40. Co-ground calcium carbonatematerial according to claim 32, comprising a ZrO₂/CeO₂ weight ratio offrom 4 to 6.5, preferably of from 4.6 to 5.7, and most preferably of5.3.
 41. Paper, paints or plastics comprising the co-ground calciumcarbonate material according to claim 19.